Devices which use disks in a variety of formats such as optical disks, opto-magnetic disks, or flexible magnetic disks are well known, but of these the hard disk drive (HDD) is widely used in electronic equipment such as video recording and playback devices and satellite navigation systems, in addition to computer systems.
The HDD stores user data in the magnetic disk upon receiving a write command from the host. In practical terms, the HDD stores the user data received from the host in an address on the magnetic disk specified by the write command. Moreover, the HDD reads out data from the magnetic disk at the address specified by a read command from the host, and transfers this to the host.
To protect user data stored in the disk from unauthorized access, many technologies are known that encrypts this user data in the disk drive.
The technology for encrypting user data is also used in some HDDs. The HDD encrypts user data received from the host, and stores this encrypted user data in the magnetic disk. Then in the read process, the HDD reads out the encrypted user data, decrypts it, and transfers it to the host.